The present invention relates generally to image-forming devices, and more particularly to a development device for use with an electrophotographic image-forming device. The present invention is suitable, for example, for a development device for an electrophotographic recording device (or printer). The "electrophotographic recording device" by which we mean is a recording device employing the Carlson process described in U.S. Pat. No. 2,297,691, as typified by a laser printer, and denotes a nonimpact image-forming device that provides recording by depositing a developer as a recording material on a recordable medium (e.g., printing paper, and OHP film). The image-forming device of the present invention is broadly applicable not only to a discrete printer, but also to various apparatuses having a recording function such as a photocopier, a facsimile unit, a computer system, word processor, and a combination machine thereof.
With the recent development of office automation, the use of electrophotographic image-forming devices such as a laser printer for computer's output devices, facsimile units, photocopiers, etc. have spread steadily. Particularly, the electrophotographic printer as an example of the electrophotographic image-forming devices features good operability, usability for a wide range of media, high cost efficiency, and high printing quality, whereby a further improvement in high-quality and high-speed printability will be expected in future. The electrophotographic image-forming device generally includes a photoconductive insulator (photosensitive drum), and follows the procedural steps of charging, exposure to light, development, transfer, fixing, and other post-processes.
The charging step uniformly electrifies the photosensitive drum (e.g., at -600V). The exposure step irradiates a laser beam or the like on the photosensitive drum, and changes the electrical potential at the irradiated area down, for example, to -50 V or so, forming an electrostatic latent image. The development step electrically deposits a developer onto the photosensitive drum using, for example, the reversal process, and visualizes the electrostatic latent image. The reversal process is a development method that forms an electric field by a development bias in areas where electric charge is eliminated by exposure to light, and deposits the developer having the same polarity as uniformly charged areas on the photosensitive drum by the electric field. The transfer step forms a toner image corresponding to the electrostatic latent image on a recordable medium. The fixing step fuses and fixes the toner image on the medium using heat, pressure or the like, thereby obtaining a printed matter. The post-processes may include charge neutralization and cleaning on the photosensitive drum from which toner has been transferred out, a collection and recycle and/or disposal of residual toner, etc.
The developer for use with the aforementioned development step can be broadly divided into a monocomponent developer using toner and a dual-component developer using toner and a carrier. The toner may use a particle prepared, for example, in such a manner that a colorant such as a dye and a carbon black, or the like is dispersed in a binder resin made of synthetic macromolecular compound, and then is ground into a fine powder of approximately 3 through 15 .mu.m. A usable carrier may include, for example, an iron powder or ferrite bead of approximately 100 .mu.m in diameter. The monocomponent developer advantageously results in (1) simple and miniature equipment due to eliminating a carrier deterioration, a toner density control, mixing, and agitation mechanisms, and (2) used toner without any waste such as a carrier.
The monocomponent developer may be further classified into a magnetic monocomponent developer that includes toner in a magnetic powder, and nonmagnetic monocomponent developer that does not include the same. However, the magnetic monocomponent developer is disadvantageous in (1) the low transfer performance due to the high content of low electrical resistant magnetic powder which hinders the increased electric charge amount, (2) the bad colorization due to its low transparent, black-color magnetic powder; (3) the low fixing performance due to the magnetic powder which requires high temperature and/or high pressure, increasing a running cost. Accordingly, the nonmagnetic monocomponent developer without these disadvantages is expected to be in increasing demand in future.
The nonmagnetic monocomponent developer commonly uses the toner having a relatively high volume resistivity (e.g., at 300 G .OMEGA..multidot.cm, etc.). In addition, the toner, as basically carries no electric charge, needs to be charged by the triboelectrificity or charge injection in the development device.
The development process employing the nonmagnetic monocomponent developer is divided into contact- and noncontact-type development processes: The contact-type development process deposits a developer on the photosensitive drum by bringing the development roller carrying the developer into contact with the photosensitive drum; and the noncontact-type development process (as may be called jumping development process) providing a certain gap (e.g., of about 350 .mu.m) between the development roller and the photosensitive drum to space them from each other, and flies the developer from the development roller to and deposits the same onto the photosensitive drum. Disadvantageously, the contact-type development process may deteriorate the developer by friction between the development roller and the photosensitive drum, and besides cause a crack of the photosensitive film, shortening the life of photosensitive body. Accordingly, the noncontact-type development process without these deteriorations has recently been highlighted.
It is significant for the noncontact-type development process employing the nonmagnetic monocomponent developer to ensure a sufficient image density by controlling the amount of toner flying from the development roller to the photosensitive drum. Thus, it is very important to form a toner thin layer while controlling its thickness on the development roller. As a typical method for regulating a toner layer thickness of toner, it has conventionally been proposed to provide an elastic blade (restriction blade) in contact with the development roller to maintain the layer thickness uniform.
The noncontact-type development device 100a employing the typical nonmagnetic monocomponent developer, as shown in FIG. 12, generally comprises a reset roller 10a, a development roller 20a, and a blade 30a. The development roller 20a and the reset roller 10a are connected with a bias power supply, and the development bias is applied from the bias power supply 50a to the development roller 20a by superposed AC voltage 54a and DC voltage 52a. The reset roller 10a, which is also called supply roller or application roller, contacts the development roller 20a and serves not only to supply toner T to the development roller 20a, but also to scrape off and remove the toner T unused for the development and remaining on the development roller 20a.
To be more specific, the reset roller 10a, which is typically made of an elastic member such as urethane foam, comes in contact with and is engaged in the development roller 20a to the depth of 0.2 through 0.5 mm, and rotates against the direction of rotation of the development roller 20a. The development roller 20a, which is, for example, a roller made of metal such as aluminum, adsorbs the charged toner on its surface in the form of the thin layer, and conveys it to a development area.
The blade 30a is brought into contact with the development roller 20a and serves to regulate the toner layer to a uniform thickness. The blade 30a may be made up of one elastic member such as urethane, or of a metal member having a contact portion made of resin with the development roller 20a. For instance, according to Japanese Patent Applications Laid-Open Nos. 8-202130 and 6-102748, when a metal member, namely rigid member, is used for the development roller 20a, the toner layer may be regulated by bringing a blade 30a made of an elastic body such as rubber into contact with the development roller; on the other hand, when a member made of an elastic body such as rubber is used for a surface of the development roller 20a, the toner layer may be regulated by bringing an end portion or non-end portion (namely midsection) of the blade 30a into contact with the development roller 20a. In order to avoid damaging the development roller 20a and the blade 30a by mitigating the accuracy in contact pressure required at the contact potion between them, these prior arts have devised to use a contact between one that is rigid and the other that is elastic. The Japanese Patent Applications Laid-Open Nos. 8-202130 and 6-102748 also disclose a surface roughness of the development roller 20a, a pressure with which the blade 30a is pressed against the development roller (blade pressure), a toner particle diameter, and other conditions for forming a toner layer.
In operation, the toner T is charged (e.g., negatively) by sliding friction among the reset roller 10a, the blade 30a, and the development roller 20a. The negatively charged toner T thereafter is fed onto a surface of the development roller 20a by the reset roller 10a, and deposited thereon by electrostatic adsorption. Subsequently, the toner layer on the development roller 20a is leveled by the blade 30a to form a thin layer having a uniform thickness of about 10 .mu.m through 40 .mu.m. The toner T, which has been conveyed to a development area where a surface of the development roller 20a is closest to the photosensitive drum 210a, flies and adhered to an electrostatic latent image on the photosensitive drum 210a with the electrical force of attraction using a predetermined voltage applied to the development area. Consequently, the latent image is visualized and developed. Next, the reset roller 10a removes the residual toner on the development roller 20a that is left in a no-image area where no latent image is formed. The development process repeats a series of these operations.
However, since a voltage having the same waveform as the development roller 20a is applied to the reset roller 10a by the development bias power supply 50a, a toner supply from the reset roller 10a to the development roller 20a varies with areas on the development roller 20a according to whether the areas were used last time for development on the photosensitive drum 210a, and would disadvantageously produce a so-called incidental image. The incidental image would disadvantageously cause an insufficient supply of the toner T to the development roller 20a, leading to a lack of density in areas once used when the same areas are used for the second or third time.
The toner T remaining in unused area increases in charge amount by passing under the blade more then once, and thus may disadvantageously lower the density due to the reduced amount of development in that area. Decrease in performance of collecting the residual toner T by the reset roller 10a is, as pointed out in Japanese Patent Application Laid-Open No. 9-54496, derived from increase in apparent rigidity due to cumulative clogs of the toner T into the urethane foam.
Moreover, the residual toner would disadvantageously film the development roller 20a, or adhere to the brush 30a. Filming is a phenomenon in which part of the toner T is deposited onto the development roller 20a by the frictional heat, mechanical crushing force, or electrostatic force that generates by the friction between a surface of the development roller 20a and the toner T.
As a result, the toner layer having a uniform thickness and charge amount cannot be formed on the development roller 20a, lowering the development performance.
In order to eliminate these disadvantages, it is conceivable to adopt a method of collecting the toner in full by deepening the contact between the development roller 20a and the reset roller 10a to facilitate mechanically scraping off the toner T. However, this method may increase a contact pressure between the development roller 20a and the reset roller 10a, and disadvantageously increase torque, crush the toner T by a repeated use, and degrade properties of the toner T by embedding external additives (e.g., silica) in the toner T. The toner T, if degraded, would reduce its fluidity and become coagulated, and thus disadvantageously be prevented from flying to the photosensitive drum, or allowed to fly in the form of lumps, deteriorating its printing quality, and particularly, its resolution.